1. Field of the Invention
The present invention relates generally to an antenna, and, more particularly, pertains to a high-frequency vertical antenna.
2. Description of the Prior Art
Those concerned with antennas have long recognized the need for a high-frequency vertical antenna including automatic band switching. The present invention fulfills this need.
The traditional prior art vertical antennas have relied on anti-resonant inductor-capacitor circuit traps placed at or near the quarter-wave current antinode points to decouple varying lengths of the available radiating structure on those bands where the total height of the vertical antenna was greater than an electrical quarter wavelength. The approach provided that the overall height of the radiating structure was typically less than a quarter wavelength at the lowest frequency of operation and the exact height was largely determined by the inductance-capacitance ratio of the traps. The usual method of providing eighty-meter resonance in vertical antennas was to utilize a high inductance coil at the top of the structure which simultaneously served as a forty-meter decoupling trap and as a loading for eighty-meter resonance. In most designs, additional loading in the form of capacity hats was used to limit the overall height of the structure to something less than one-eighth wavelength on the lowest frequency. The physical height of the active radiating sections was usually less than a quarter wavelength because of the inductive reactance of the several decoupling traps at frequencies below the frequencies to which the decoupling traps were tuned.
The prior art vertical antennas have had a number of limitations. First, the active antenna height on all but the highest frequency band was necessarily less than one quarter wavelength resulting in a radiation resistance which progressively decreased from a high impedance on the highest frequency of operation to a few ohms on the lowest frequency of operation. Second, the use of numerous traps and other loading devices increased the system Q and unnecessarily restricted the band width, especially on the mid-range HF (high frequency) frequencies where the active radiator height would be less than that required for unloaded resonance operation. Third, from a mechanical viewpoint, the use of numerous traps and loading devices in the upper sections of the vertical antenna made for a relatively unstable and heavy structure which required heavy and expensive construction for a freestanding wind survival rating. Fourth, a further difficulty had to do with the ease of adjustments for resonance at the desired frequencies in the low HF frequencies. Inasmuch as adjustment in the past for these frequencies had to be made in the upper sections of the antenna, the entire vertical antenna had to be removed from its mounting and brought to ground level for the slightest readjustment. This was a particularly inconvenient feature of operation as the effective operating band width of the vertical antenna was generally less than twenty percent of the authorized band spectrum.
The present invention provides a vertical antenna that overcomes all the disadvantages of the prior art vertical antennas.